In recent years, there has been an everincreasing amount of manufacturing to a very high degree of dimensional precision. As a result, there is an urgent need for simple non-contact means for measuring minute values of dimensions with high accuracy, which can be put to practical use in an actual manufacturing environment rather than in the controlled environment of a laboratory. Such dimensional measurement is primarily concerned with the measurement of distances between edges upon a surface of an object, e.g. outer edges of a surface or the edges of raised or recessed portions of a surface.
One method which has been proposed in the prior art to accomplish this is to direct a beam of light onto the surface concerned, and to arrange one or more photo-sensors such as to detect the intensity of the resultant light reflected from the surface. The body under measurement is mounted upon a movable stage, e.g. movable in an X-Y manner, for example as shown in FIG. 1 of the drawings in which a body 14 is mounted on a movable stage 16, with a beam of light 10 being focussed to form a small spot on the upper face of body 14 by a lens 12. Photo-sensors 18 and 20 are disposed with respect to the position of incidence of the light beam 10 such as to receive reflected light from the surface. With such an arrangement, the intensity of reflected light sensed by sensors 18, 20 is a minimum when light beam 10 falls upon a portion of the surface of body 14 which is flat and aligned perpendicular to the direction of incidence of light beam 10, while the intensity of the sensed reflected light is a maximum when the light beam 10 is incident upon a portion of the surface of body 14 which is substantially inclined with respect to the direction of incidence of light beam 10, i.e. at an edge of object 14. In this way, the position of an edge can be detected as a change in the intensity of the reflected light sensed by sensors 18, 20 as body 14 is moved with respect to light beam 10. However such an arrangement has various fundamental disadvantages. Firstly, it is necessary to position the photo-sensors very precisely with respect to the position of incidence of light beam 10 on object 14. Secondly, the method is based upon changes in the absolute magnitude of the reflected light, which will of course vary considerably between an edge portion which is relatively shallow and one which is relatively deep. Thirdly, due to the use of mechanical movement of the body under measurement, in order to scan light beam 10 over the body surface, irregularities of movement of the body will produced corresponding errors of measurement, so that such a method cannot be used for measurement of dimensions to a very high degree of accuracy.
There is therefore a requirement for a non-contact form of apparatus for measuring minute distances between edges of a surface on an object which will overcome the disadvantages of the prior art as described above, yet which will be simple and practical. Such an apparatus is disclosed by the present invention.